A trend in the semiconductor arts is to fabricate integrated circuits incorporating devices of various technologies. An example of this is BICMOS integrated circuits which include bipolar devices, P channel MOS devices and N channel MOS device. To successfully merge these technologies, device structures are required that are compatible to both bipolar and MOS devices. A desirable BICMOS structure should include a buried layer for the BICMOS devices as well as N wells and P wells for the N channel and P channel MOS devices respectively.
One method of fabricating a BICMOS structure includes forming an N+ buried layer and implanting P+ dopant into the buried layer. Following the implantation, an N type epitaxial silicon well layer is grown on the buried layer and the P type dopant diffuses into the well layer to form an N channel therein. This method has been relatively unsuccessful because the P type dopant diffusion is uncontrolled. The dopant will not diffuse only up into the well layer but diffuses in all directions thereby providing an unsatisfactory structure.
Another method of forming P type wells over an N type buried layer includes forming an N type buried layer and growing an epitaxial silicon well layer thereon. Once the well layer has been grown, a screen oxide layer is formed thereon and P type dopant is implanted into the epitaxial silicon well layer. This method is unsatisfactory because it is not feasible to implant the P dopant completely through the well layer to the buried layer. This method again yields devices having poor performance.
In view of the above, it would be highly desirable to have a method of forming oppositely doped wells over a buried layer that would yield high quality semiconductor devices.